When looking at a 12-lead ECG, there are a few logistics that must be known. First of all, the standard 12-lead ECG is a 10 second strip. The bottom one or two lines will be a full "rhythm strip" of a specific lead spanning the whole 10 seconds of the ECG. Other leads will only span about 2.5 seconds.
Each ECG is divided by large boxes and small boxes to help measure times and distances. Each large box is 0.20 seconds. There are five small boxes in each large box, thus each small box is 0.04 seconds. The below images depicts each of these:
If the entire ECG is 10 seconds, then there must be 50 large boxes (0.20 seconds X 50 large boxes). Each small box is also exactly 1 mm in length and so one large box is 5 mm. In general, when measuring amplitudes of waves or complexes, the units are expressed in millimeters (mm) and when measuring lengths for intervals, the units are expressed in time (seconds or milliseconds).
If each small box is equal to 0.04 seconds or 1 mm, then the standard ECG speed is 1 mm per 0.04 seconds or 25 mm per second.
The standard approach to reading an ECG includes in this order:
Examining the rate
Examining the rhythm
Examining the axis, intervals and segments
Examining everything else.